Administrator Guide

traffic assumes the role of the Primary Peer. If the original Primary Peer is restored, the VLT peer reassigned as the Primary Peer retains
this role and the other peer must be reassigned as a Secondary Peer. Peer role changes are reported as SNMP traps.
RSTP and VLT
VLT provides loop-free redundant topologies and does not require RSTP.
RSTP can cause temporary port state blocking and may cause topology changes after link or node failures. Spanning tree topology
changes are distributed to the entire layer 2 network, which can cause a network-wide flush of learned MAC and ARP addresses, requiring
these addresses to be re-learned. However, enabling RSTP can detect potential loops caused by non-system issues such as cabling errors
or incorrect configurations. To minimize possible topology changes after link or node failure, RSTP is useful for potential loop detection.
Configure RSTP using the following specifications.
The following recommendations help you avoid these issues and the associated traffic loss caused by using RSTP when you enable VLT on
both VLT peers:
• Configure any ports at the edge of the spanning tree’s operating domain as edge ports, which are directly connected to end stations
or server racks. Disable RSTP on ports connected directly to Layer 3-only routers not running STP or configure them as edge ports.
• Ensure that the primary VLT node is the root bridge and the secondary VLT peer node has the second-best bridge ID in the network. If
the primary VLT peer node fails, the secondary VLT peer node becomes the root bridge, avoiding problems with spanning tree port
state changes that occur when a VLT node fails or recovers.
• Even with this configuration, if the node has non-VLT ports using RSTP that you did not configure as edge ports and are connected to
other Layer 2 switches, spanning tree topology changes are still detected after VLT node recovery. To avoid this scenario, ensure that
you configure any non-VLT ports as edge ports or disable RSTP.
VLT Bandwidth Monitoring
When bandwidth usage of the VLTi (ICL) exceeds 80%, a syslog error message (shown in the following message) and an SNMP trap are
generated.
%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-
channel 25)
crosses threshold. Bandwidth usage (80 )
When the bandwidth usage drops below the 80% threshold, the system generates another syslog message (shown in the following
message) and an SNMP trap.
%STKUNIT0-M:CP %VLTMGR-6-VLT-LAG-ICL: Overall Bandwidth utilization of VLT-ICL-LAG (port-
channel 25)
reaches below threshold. Bandwidth usage (74 )VLT show remote port channel status
VLT and High Availability
High availability (HA) support on VLT ensures seamless and uninterrupted flow of VLT features during RPM failure (failover).
When RPM failover happens, the new active RPM triggers a new VLT registration to its VLT peer. It ensures that the VLT node with new
active RPM receives all the VLT information from its VLT peer. When the standby RPM performs the check-in and registration function
(with the active RPM), the latter performs a bulk synchronization of all the peer VLT information. The existing CLI configuration
synchronized to the standby RPM ensures that the local VLT configurations are always available at standby RPM as well. The VLT backup
link functionality is also modified to manage the two management interfaces in a dual RPM. A virtual management IP must be configured
on the dual RPM VLT node to maintain uninterrupted VLT backup functionality. For more information, refer to “VLT backup link” section in
the Configuration Notes
VLT and IGMP Snooping
When configuring IGMP Snooping with VLT, ensure the configurations on both sides of the VLT trunk are identical to get the same
behavior on both sides of the trunk.
When you configure IGMP snooping on a VLT node, the dynamically learned groups and multicast router ports are automatically learned on
the VLT peer node.
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Virtual Link Trunking (VLT)